The present invention relates to a substrate alignment system and method thereof. The substrate alignment system is commonly used in a lithographic projection apparatus (or wafer stepper) in fabricating semiconductor devices. One of the function of the stepper is to “print” electronic circuit information onto a substrate by lithographic techniques. The lithographic projection apparatus may be a stepping apparatus or a step-and-scan apparatus.
In a stepping apparatus, the mask pattern is imaged in one run on an integrated circuit (“IC”) area of the substrate. Subsequently, the substrate is moved with respect to the mask in such a way that a subsequent IC area will be situated under the mask pattern and the projection lens system and the mask pattern is imaged on the subsequent IC area. This process is repeated until all IC areas of the substrate are provided with a mask pattern image.
In a step-and-scan apparatus, the above-mentioned stepping procedure is also followed, but the mask pattern is not imaged in one run but via scanning movement. During imaging of the mask pattern, the substrate is moved synchronously with the mask with respect to the projection system and the projection beam, taking the magnification of the projection system into account. A series of juxtaposed partial images of consecutively exposed parts of the mask pattern is imaged in an IC area. After the mask pattern has been completely imaged in an IC area, a step is made to a subsequent IC area. Currently, the step-and-scan apparatus is more widely used in the semiconductor industries since scanning uses the lens more effectively than static exposure of the entire area. An example of a step-and-scan apparatus is described in U.S. Pat. Nos. 6,452,662 and 6,297,876, which are incorporated by reference. A method of using a scan-step system is described in, “Extended ATHENA™ Alignment Performance and Application for the 100 NM Technology Node,” by Ramon Navarro et al., of ASML, De Run 1110, 5503 LA Veldhoven, The Netherlands, presented at SPIE, the 26th Annual International Symposium on Microlithography, Feb. 25-Mar. 2, 2001, Santa, Clara, Calif., which is incorporated by reference.
Some of the important considerations in lithography are (1) critical dimension (“CD”) control or imaging, and (2) overlay. The CD control defines how accurately the line width of structures formed on the substrate can be controlled. The CD control strongly influences the final performance, e.g., speed and power, of the semiconductor devices that have been fabricated. The overlay relates to the repositioning accuracy of successive exposures. Since the circuitry is patterned onto the substrate by repeatedly exposing the substrate to light and processing it layer-by-layer, it is important that the substrate is positioned at nearly the same position as in the previous processes. Accordingly, the overlay strongly influences the density of semiconductor devices that may be fabricated. Various types of alignment marks are used to properly align the substrate with minimum overlay.